Beyond 8% ultrathin kesterite Cu2ZnSnS4 solar cells by interface reaction route controlling and self-organized nanopattern at the back contact

Highly efficient, ultrathin (~400 nm) pure sulfide kesterite Cu2ZnSnS4 (CZTS) solar cells have been realized by interface reaction route controlling and self-organized nano-pattern at the back contact. The Al2O3 intermediate layer introduced at the Mo/CZTS interface can effectively inhibit the detrimental interfacial reaction between CZTS and Mo in the initial stage of sulfurization, and then turns into a self-organized nanopattern yielding a nanoscale opening for electrical contact. With this interface modification, the traditional issues of phase segregation (secondary phases) and voids at the back contact region can be well addressed, which greatly improves uniformity and reduces back contact recombination. As a result, this interface modification not only leads to beyond 8% ultrathin CZTS solar cells but also yields two certificated world record efficiencies: 9.26% for 0.237 cm2 small area and 7.61% for 1 cm2 standard kesterite CZTS solar cells (normal thickness). Solar cells made from inexpensive, non-toxic elements can reach record efficiencies with judicious application of aluminium oxide (Al2O3). Ultrathin films of kesterite semiconductors composed of copper–zinc–tin–sulfur (CZTS) atoms achieve high light absorption but suffer from poor power conversion. Researchers at the University of New South Wales, Australia, led by Xiaojing Hao, found that inserting nanometer-thin dielectrics such as aluminium oxide between a CZTS film and a reflective back contact raised conversion efficiency in two ways. The initially continuous insulating film suppressed decomposition reactions, which cause high device resistance and recombination velocity. Then, during the formation of the molybdenum–sulfur back contact layer, the aluminium oxide self-organizes into a nanoporous film. The openings in the dielectric enable direct connection of the CZTS film to the electrical circuit, boosting conversion efficiency by 1–2% over other approaches. Over 8% efficient ultrathin kesterite Cu2ZnSnS4 (CZTS) solar cells have been developed by interface reaction route controlling and self-organized nanopattern at the back contact. An ultrathin dielectric Al2O3 intermediate layer is inserted into the Mo/CZTS interface to inhibit the detrimental interfacial reaction between CZTS and Mo, and then turns into a self-organized nanopatterning with opening for electrical contact. The typical issues of phase segregation and voids at the back contact region therefore can be addressed, which reduces back contact recombination and improves the device performance. A 7.6% efficiency world record for 1 cm2 CZTS solar cells also has been achieved by this interface modification.